In an internal combustion engine the inlet and exhaust valves are normally controlled by means of a cam system that controls the opening of the valves (i.e. to push the valves inside the respective cylinders) and mechanical springs to control the closing of the valves (i.e. to push the valves against their seats). In other words, a mechanical spring is coupled to the stem of each valve and pushes the valve towards the closed position (i.e. against its respective seat) and a cam attached to a cam shaft is mechanically coupled to cyclically push the valve towards the open position against the action of the mechanical spring.
The mechanical springs that cause the valves to close must be dimensioned so as to be able to close the valves in an interval of time that is defined by the maximum speed the engine can reach; consequently, at all other engine speeds the mechanical springs are over-sized and their cycle of compression and expansion inevitably results in a waste of energy which reduces the energy efficiency of the engine. In standard motor vehicle engines that do not reach high maximum speeds (generally not more than 5000 rpm for diesel engines and 7000 rpm for petrol engines) the mechanical valve springs use only a moderate amount of energy; however, in high-performance engines, which must necessarily reach very high speeds (of over 10,000 rpm) in order to deliver high power, the mechanical valve springs use a significant amount of energy. The use of pneumatic springs instead of the conventional mechanical springs has been proposed as a means of reducing the energy used by the valve springs. In a pneumatic spring the elastic force is generated by the compression of a fluid (typically air) rather than by the deformation of an elastic member as is the case with a mechanical spring and in a pneumatic spring it is thus possible to adjust the elastic force generated by the pneumatic spring by adjusting the pressure of the fluid inside said pneumatic spring; consequently, using pneumatic springs to control the closing of the valves makes it possible to adjust the elastic force generated by the pneumatic springs according to the engine speed and significantly reduce the amount of energy that is wasted in actuating the valves.
Patent applications GB209035 and FR2364328 describe a valve of an internal combustion engine provided with a return device comprising a pneumatic spring and a mechanical coil spring.
Patent application DE3808542A1 describes a valve of an internal combustion engine provided with a return device comprising a pneumatic spring and a mechanical coil spring, which is dimensioned to develop a limited elastic force suitable for a moderate engine speed (low number of rpm). In the low rpm range, a chamber of the pneumatic spring is connected to the atmosphere and the return force of the valve is generated exclusively by the mechanical coil spring; in the high rpm range, the chamber of the pneumatic spring is connected by means of a pressure regulator to a pneumatic accumulator containing compressed air and the return force of the valve is generated by both the mechanical coil spring, and, primarily, by the pneumatic spring.
Patent application DE4214839A1 describes a valve of an internal combustion engine provided with a return device comprising a pneumatic spring and a mechanical coil spring, which develops a limited elastic force and has the safety function of guaranteeing engine operation at minimum speed even in the event of a pneumatic spring failure. The chamber of the pneumatic spring is maintained under pressure by means of a pressurized tank that is constantly connected to said chamber by means of a feed conduit having a reduced cross-sectional area.
Patent EP1381757B1 describes a valve of an internal combustion engine provided with a return device comprising a pneumatic spring and a mechanical coil spring, which produces a limited elastic force and has the purpose of enabling the engine to run at minimum speed even in the event of a pneumatic spring failure.
Patent application EP1143115B1 describes a valve of an internal combustion engine provided with a return device comprising a pneumatic spring and a mechanical coil spring, which produces a limited elastic force suited to a low engine speed (low rpm range). In the low rpm range, a chamber of the pneumatic spring is connected to the atmosphere via a first box-type solenoid valve and the return force of the valve is generated exclusively by the mechanical coil spring; in the high rpm range, the chamber of the pneumatic spring is connected to a pressure source via a second box-type solenoid valve and the return force of the valve is generated by both the mechanical coil spring, and, primarily, by the pneumatic spring.
The pneumatic valve control systems described above are complex to produce and offer limited reliability.